A measure of velocity relative to the speed of sound for a given medium. For instance, Mach 2.0 is equal to twice the speed of sound for a given medium. In dry air, for example, at 0° Celsius, and at one atmospherepressure, sound travels at approximately 331.6 meters per second, so mach 2.0 in this case would be about 1,194 km/hr. Usually utilized to express the speed of an aircraft relative to the speed of sound in air.